I assume movie versions? Good question. Groot's limbs and reach could be an issue, but Venom is a great deal faster than him. No telling how the symbiote would work (if at all) to Groot's physiology and vice versa.

Both have the ability to grow tendrils, they are both roughly equals in strength, and they both have a healing factor (Venom's HF appears to be faster while Groot's is possibly more powerful). I think I might side with Venom in this as he is quite a bit faster and more agile. Groot looked pretty slow against both Gamora and Drax.